3.2 & 7.5 Proteins

Pp 50 – 51 & 206 - 210

Pp 15 & 66 - 68

ProteinsProteins are polymers of amino acids Each has a unique 3D shapeAmino acid sequences varyProteins are major component of cell partsThe provide:

support and structural componentsstorage of amino acids

Several types of proteins are identified: receptor, contractile, defense, enzymes, structural

Building Blocks: Amino Acids There are 20 different types of amino acids

All have this general formular

The R group is a variable group

Polar vs. Non-polar Amino Acids

Some amino acids are polar while some are non-polar

This property causes proteins to fold into varied shapes

Significance of polar & non-polar amino acids

polar amino acids non-polar amino acids

hydrophiliccan make hydrogen

bonds

found in hydrophilic channels & parts of proteins projecting from membranes

found on surface of water-soluble proteins

hydrophobicforms van der Waals

(hydrophobic interactions) with other hydrophobic amino acids

found in proteins in interior of membranes

found in interior of water-soluble proteins

Peptide Bonds

Proteins are formed by condensation

A peptide bond is formed

Four levels of protein structure

Primary StructurePrimary structure is a

chain of amino acids number & unique

sequence of amino acids determine the properties of primary structure

each position is occupied by one of 20 different amino acids

sequence of amino acids is determined by DNA sequence in genes

linked by peptide bonds

Secondary structureformed by interaction

between amino and carboxyl i.e. -NH and -C=O groups

weak hydrogen bonds are formed between – H & = O

there are two types: a-helix and b-sheet

α- helix formed / polypeptide coils up e.g. sheep wool

β- pleated sheet formed e.g. silk in spider webregular repeated folding of

amino acid chainsecondary structure is

stabilized by hydrogen bonds

Tertiary structureThese are globular proteins

with irregular conformationtertiary structure is the

folding up of the polypeptide chain, secondary structure or alpha helix

it gives three dimensional globular shape i.e. shape of active site

the structure is stabilized by disulphide bridges, hydrogen, ionic& hydrophobic bonds

tertiary structure used as enzymes to catalyze biochemical reactions

Quaternary structuremade of several

polypeptide subunits joined together

they maybe conjugated proteins i.e. proteins which combine with a prosthetic group (non-protein molecules)

prosthetic groups includes: metals e.g. iron in haemoglobin, nucleic acids as in ribosomes , carbohydrates as in glycoprotein or lipids as in glycolipids

Denaturation of Proteinschange in

protein’s usual regular structure due to: High tempChange in pHAddition of

organic solvent (alcohol, acetone)

These factors break the bonds that stabilize the structure

Protein Functions Function Examples

StructureTransport

Enzymes

Movement

HormonesAntibodiesstorage

– collagen/keratin/fibrin – myoglobin/hemoglobin, bind

& transport oxygen – lysozyme, speeding up

metabolic reactions – actin (and myosin tropomyosin

(and troponin)– insulin, regulate blood glucose – immunoglobulin– albumin in egg, casein in

milk

Difference between Fibrous and Globular Proteins

fibrous proteins are long and narrow strands or sheets whereas globular proteins are rounded, spherical or ball shaped

fibrous protein made of repetitive amino-acid sequences whereas globular proteins are made of irregular amino acid sequences

fibrous proteins are usually insoluble in water whereas globular proteins usually soluble in water

globular protein more sensitive to changes in pH, temperature & salt than fibrous proteins

fibrous proteins have structural or support functions (roles) whereas globular proteins have metabolic functions such as catalysis & transport function

Examples of fibrous proteins: keratin/fibrin/collagen/actin/myosin/silk protein

Examples of globular protein: insulin/immunoglobulin/hemoglobin/amylase

Revision QuestionsExplain how polar and non-polar amino acids help channel

proteins and enzymes carry out their functions. [5]Distinguish between fibrous and globular proteins with

reference to one example of each protein type. [6]Explain primary structures and tertiary structures of a

protein. [3]

Draw the structure of a generalized dipeptide. [4]Draw the basic structure of an amino acid, and label the

groups that are used in peptide bond formation. [4]List four functions of membrane proteins. [4]Giving a named example of each, state four functions of

proteins. [8]